The present invention relates to compositions useful as transmission fluids, and particularly as fluids for continuously variable transmissions.
Continuously variable transmissions (CVT) represent a radical departure from conventional automatic transmission. The bush belt version of the CVT was invented by Dr. Hub Van Doorne, and since its introduction, many cars have been equipped with the push belt CVT system. CVTs are manufactured by Van Doorne's Transmissie VB of Tilburg, the Netherlands. A more detailed description of such transmissions and belts and lubricants employed therein is found in European Patent Application 753 564, published Jan. 15, 1997, as well as references cited therein. In brief, a belt and pulley system is central to the operation of this type of transmission. The pulley system comprises a pair of pulleys with a V-shaped cross-section, each consisting of a moveable sheave, a fixed sheave, and a hydraulic cylinder. Between the pulleys runs a belt, which consists of a set of metal elements held together by metal bands. In operation, the driving pulley pushes the belt to the driven pulley, thereby transferring power from the input to the output. The transmission drive ratio is controlled by opening or closing the moveable sheaves so that the belt rides lower or higher on the pulley faces. This manner of operation permits continuous adjustment of gear ratio between the input and output shafts.
It has become clear from commercial use of the CVT that the fluids used in the CVT are just as important as the mechanical design for satisfactory operation. The lubricant must fulfill several functions: to lubricate the metal belt in its contacts with the pulley assembly, the planetary and other gears, the wet-plate clutches, and the bearings; to cool the transmission; and to carry hydraulic signals and power. The hydraulic pressure controls the belt traction, transmission ratio, and clutch engagement. The lubricant must provide the appropriate degree of friction between the belt and pulley assembly, to avoid the problem of slippage on one hand, and binding on the other, all the while providing protection to the metal surfaces from pitting, scuffing, scratching, flaking, polishing, and other forms of wear. Accordingly, the fluid should maintain a relatively high coefficient of friction for metal/metal contact, as well as exhibiting a suitable degree of shear stability.
Copending U.S. patent application Ser. No. 08/500,810, Sumiejski et al., filed Jul. 10, 1995, which is equivalent to EP 0 753 564 referred to above, published Jan. 15, 1997, discloses a shear stable lubricating/functional fluid composition, comprising an oil of lubricating viscosity, 1-15% by weight of the metal salt of an organic acid, and 1-25% viscosity modifier, wherein the composition has certain defined viscosity. Other components in the additive package include a metal dialkyl dithiophosphate, sulfur containing friction modifiers, dialkyl phosphites, and fatty amides.
European Application 761 805, Mar. 12, 1997, discloses a lubricating/functional fluid which comprises an oil of lubricating viscosity, 2,5dimercapto-1,3,4-thiadiazole or a derivative thereof and an antifoam agent. The composition may include phosphoric acid. Friction modifiers are included in the compositions in the amounts of 0.1-10 weight percent and may be a single friction modifier or mixtures of two or more. Friction modifiers also include metal salts of fatty acids. Preferred cations are zinc, magnesium, calcium, and sodium and any other alkali, or alkaline earth metals may be used. The salts may be overbased by including an excess of cations per equivalent of amine [sic; acid?]. Zinc salts are added in amounts of 0.1-5 weight percent to provide antiwear protection. The zinc salts are normally added as zinc salts of phosphorodithioic acids.
U.S. Pat. No. 4,792,410, Dec. 20, 1988, Schwind et al., discloses a lubricant mixture suitable for a manual transmission fluid, comprising a boronated overbased alkali metal or alkaline earth metal salt, a friction modifier or mixture of friction modifiers, and an oil of lubricating viscosity.